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Aqueous hydrogen peroxide decomposes into oxygen gas and water. The reaction is normally very slow but is catalysed by solid manganese(IV) oxide.
$$2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$$
You are to plan an experiment to investigate how the rate of the catalysed decomposition of aqueous hydrogen peroxide depends on its concentration.
(a) The rate of decomposition depends on the number of hydrogen peroxide molecules present in a given volume of solution.
(i) Use this information to predict and explain how the rate of decomposition of the hydrogen peroxide depends on the concentration.
Prediction .........................................................................................................................
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Explanation ......................................................................................................................
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(ii) Display your prediction in the form of a sketch graph below.
(b) An approximate method for the determination of the rate of decomposition of the hydrogen peroxide is to measure the time taken to collect a fixed volume of oxygen. The volume is kept the same throughout a series of experiments. The rate of the reaction can be represented by the reciprocal of the time taken.
Rate of reaction $\propto$ 1/time taken
In the experiment you are about to plan identify the following.
(i) the independent variable ............................................................................................................
(ii) the dependent variable ...............................................................................................................
(c) Draw a diagram of the apparatus you would use in the experiment. Your apparatus should use only standard items found in a school or college laboratory and show clearly the following
(i) how the volume of the oxygen will be collected and measured,
(ii) how you will make sure that none of the oxygen is lost.
Label each piece of apparatus used, indicating its size or capacity.
(d) Using the apparatus shown in (c) design a laboratory experiment to test your prediction in (a).
In addition to the standard apparatus present in a laboratory you are provided with the following materials.
• 2.00 mol dm$^{-3}$ aqueous hydrogen peroxide
• a supply of manganese(IV) oxide
(i) Complete the table below to show how you would prepare five solutions of aqueous hydrogen peroxide. Make sure that the correct units are recorded.
[Table_1]
(ii) Give a step-by-step description of how you would use your apparatus shown in (c) to carry out one complete experiment.
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(e) State a problem which might be experienced by someone having to carry out these experiments alone.
...................................................................................................................
(f) Draw a table with appropriate headings to show the data you would record when carrying out your experiments and the values you would calculate in order to construct a graph to support or reject your prediction in (a). The headings should include appropriate units.
570
556
500
(a) The results of the series of titrations are recorded below.
[Table_1]
A | B | C | D | E
expt. No. | volume of 0.10 mol dm^{-3} NaOH reacting with 10.0 cm^3 of the water layer / cm^3 | volume of 0.020 mol dm^{-3} NaOH reacting with 25.0 cm^3 of the ether layer / cm^3
1 | 24.3 | 18.6
2 | 22.5 | 17.3
3 | 20.3 | 15.6
4 | 18.8 | 13.1
5 | 16.3 | 12.5
6 | 13.8 | 10.6
7 | 10.3 | 7.9
8 | 6.8 | 6.9
9 | 5.0 | 3.8
10 | 2.5 | 1.9
Process the results in the table to calculate the concentration of the succinic acid in each layer.
Record these values to three significant figures in the additional columns of the table.
Label each column, including units and an expression to show how your values are calculated.
You may use the column headings A to E in your expression.
(b) Present the concentration of the succinic acid in each layer in graphical form. Draw the line of best fit.
[Graph]
(c) Circle on the graph any point(s) you consider to be anomalous. For any point circled on the graph suggest an error in the conduct of the experiment that might have led to this anomalous result.
(d) (i) Determine the value of K from your graph. Mark clearly on the graph any construction lines and show clearly in your calculation how the intercepts were used in the calculation of the slope.
(d) (ii) By considering the data you have processed and the graph you have drawn, decide if the experimental procedure described is suitable for the determination of the Partition Coefficient, K. Explain your reasoning.
(e) In the experimental procedure a small volume of water was added to the diethyl ether prior to the titration with aqueous sodium hydroxide. The flask was constantly shaken during the titrations. What was the purpose of this technique?
(f) Using a burette, the error associated with a titration depends on the value of the titre. Comment on the magnitude of the titres recorded in the table in (a) and indicate, with reasons, which have the highest error.
598
505
582
